Prior research has identified the integration of marketing with research and development (R&D) as a key success factor for new product development (NPD). However, prior work has not distinguished the sales and marketing functions, even though they are distinctive departments within an organization. Therefore, the authors extend prior research and examine the effect of cross-functional cooperation among sales, marketing, and R&D on NPD performance across multiple stages of the NPD process. The authors use multiple-informant data from 424 sales, marketing, and R&D managers as well as project leaders of 106 NPD projects to test several hypotheses. The results show that the cooperation between sales and R&D and between sales and marketing has a significant, positive effect on overall NPD project performance beyond marketing–R&D cooperation. The authors also find that the effect of cross-functional cooperation among sales, marketing, and R&D on overall NPD project performance varies across stages of the NPD process. More specifically, the authors find that sales–R&D cooperation in the concept and product development stages is critical for greater new product success. Sales–marketing cooperation is important in the concept development stage but has surprisingly less impact in the implementation stage.